A 3.9mW 25-electrode reconfigured thoracic impedance/ECG SoC with body-channel transponder

Recently, wearable heart monitoring systems have been developed for cardiovascular-related disease [1] with wearable body sensor network (WBSN) [2–3]. The WBSN introduced in [3] monitored ECG at maximum 48 points, and transferred data using arrayed inductive link for cm-range wireless inter-connectivity. However, most of the previous attempts were limited to sense only ECG signals at limited points [2] on the body with limited network coverage [3]. Thoracic impedance variance (TIV) from the change of aortic blood volume and velocity at each cardiac cycle provides important hemodynamic information (stroke volume, cardiac output). Combined with ECG signals from more than 6 points, it enables the early detection of abnormal symptoms of pandemic diseases like hypertension and heart failure so that the patients can take prophylactic measures [6]. In spite of its importance, the TIV detection was not realized in WBSN due to its requirement of high impedance (≪0.2Ω) detection sensitivity which needs to detect AM signal with modulation depth as low as less than 3%. A pure single tone sinusoidal current signal at 1kHz–100kHz [6] is required to realize such a high sensitivity, and only a bulky implementation was reported so far [7]. In this paper, we report a 3.9mW low power SoC with body-channel-transceiver (BCT), which can detect TIV (0.1Ω) and ECG (up to 8 points) concurrently. The chip is integrated on a 4-layer fabric circuit board with thin flexible battery as a poultice-like plaster. In addition, it can reconfigure the 25-electrode array and optimize them in-situ to automatically consider the user dependency of the TIV/ECG signals. The recorded data is transmitted at 1Mbps through body-channel-communication (BCC) [8] with duty cycle modification to extend battery life time and enlarge the network coverage.